大型火山爆發造成了最嚴重的大滅絕

 原文網址：https://www.tohoku.ac.jp/en/press/volcanic_eruption_caused_largest_mass_extinction.html

大型火山爆發造成了最嚴重的大滅絕

日本、美國和中國的研究人員表示他們找到了更加確定的證據，顯示史上最嚴重的生物大滅絕是由火山造成。這項研究探討了兩次分開的噴發事件：其中一個對研究人員來說是從未聽聞的，另一個則造成了海洋和陸地許多地方的生物走向滅絕。

圖一. 團隊發現中國南部和義大利在2億5200萬年前形成的沉積岩中有高濃度的蒄―汞。成對的蒄―汞是形成大型火成岩區域的火山多次活動的產物。他們表示這些活動造成的環境變遷可能使得許多陸地和海洋的動物消失。來源：海保邦夫

自從原始動物從四億五千萬年至六億年前開始演化出眾多分支以來，總共發生了五次生物大滅絕。其中第三次是規模最大的滅絕事件，科學家認為原因是西伯利亞暗色岩噴發所導致，這種大面積的火山岩也被稱為大型火成岩區域。但是時至今日噴發和大滅絕之間的關係仍然沒有完全釐清。

富含汞的沉積岩是大型火山事件的指標，在二疊紀末可以發現許多這類沉積岩。這些岩石的形成地點包括了內陸、淺海、大洋中央，不過要如何解讀它們目前還是沒有定論。汞的來源可能是從火山噴出之後直接從大氣沉降下來，另一種原因則是陸地或植被遭到破壞，稱為「陸域生態系擾動」的事件發生時，陸地的有機物遭到氧化而釋放出汞，接著由河川運送到沉積物裡面。

最嚴重的大滅絕事件發生在2億5200萬年前左右的二疊紀末期。這次大滅絕的特徵是動物類型的變化：古生代擁有多樣化的爬蟲類，以及腕足類和三葉蟲等海洋動物，事件之後則轉變成中生代的恐龍以及軟體類等海洋動物。大約有90%的物種在二疊紀末就此消失。

東北大學的名譽教授海保邦夫領導的研究團隊，探討了引發這起最嚴重的滅絕事件的可能原因。他們從中國南部和義大利採集了沉積岩樣品，接著分析其中的有機分子和汞(Hg)含量。結果發現有兩道分開的蒄(coronene)―汞高峰，分別可以對應到最初的陸域生態系擾動，以及接著在海洋和陸地發生的大滅絕。

「我們相信這些物質是大型火山爆發的產物，因為蒄的異常值要由溫度相當高的燃燒作用來產生。」海保教授表示。「高溫的岩漿或者小行星和彗星撞擊才能製造出這麼大量的蒄。」

「以火山來說，蒄的形成可能是因為高溫的熔岩流燃燒了活著的生物以及變成化石的有機物，或是岩漿(岩床)水平入侵到煤炭沉積物和石油裡面。這兩道蒄―汞的豐富程度不同顯示了相較於陸域生態系，對海洋生態系造成擾動的全球變遷程度要來得更大。兩次火山事件之間相隔了大約數萬年。」

巨型火山爆發會在平流層產生硫酸氣膠，並且排放二氧化碳到大氣當中，這兩者都會造成全球氣候變遷。科學家認為如此迅速的氣候變遷是陸地和海洋生物死亡的原因，如圖一所示。

蒄是種相當稠密的六環多環芳香烴，相較於小型的芳香烴來說需要更加大量的能量才能合成，因此高溫的火山燃燒過程才能形成大量的蒄。這代表側向入侵的岩漿使得沉積岩裡的碳氫化合物在高溫下燃燒，形成的二氧化碳和甲烷使得壓力變高，最終造成火山爆發，並引起全球暖化與大滅絕。高濃度的蒄―汞是第一次有證據顯示了火山燃燒碳氫化合物造成了全球暖化，促進大滅絕的發生。

海保的團隊正在研究其他的滅絕事件，期望可以更進一步地瞭解它們背後的成因以及發生過程。

 

Large volcanic eruption caused the largest mass extinction

Researchers in Japan, the US and China say they have found more concrete evidence of the volcanic cause of the largest mass extinction of life. Their research looked at two discrete eruption events: one that was previously unknown to researchers, and the other that resulted in large swaths of terrestrial and marine life going extinct.

There have been five mass extinctions since the divergent evolution of early animals 450 - 600 million years ago. The third was the largest one and is thought to have been triggered by the eruption of the Siberian Traps - a large region of volcanic rock known as a large igneous province. But the correlation between the eruption and mass extinction has not yet been clarified.

Sedimentary mercury enrichments, proxies for massive volcanic events, have been detected in dozens of sedimentary rocks from the end of the Permian. These rocks have been found deposited inland, in shallow seas and central oceans, but uncertainty remains as to their interpretation. Mercury can be sourced from either direct atmospheric deposition from volcanic emissions and riverine inputs from terrestrial organic matter oxidation when land/plant devastation - referred to as terrestrial ecological disturbance - occurs.

The largest mass extinction occurred at the end of the Permian - roughly 252 million years ago. This mass extinction was marked by the transition from the divergence of the Paleozoic reptiles and marine animals like brachiopods and trilobites to Mesozoic dinosaurs and marine animals such as mollusks. Approximately 90% of species disappeared at the end of the Permian.

Current professor emeritus at Tohoku University, Kunio Kaiho led a team that looked into possible triggers of the largest mass extinction. They took sedimentary rock samples from two places - southern China and Italy - and analyzed the organic molecules and mercury (Hg) in them. They found two discrete coronene-Hg enrichments coinciding with the first terrestrial ecological disturbance and the following mass extinction in both areas.

"We believe this to be the product of large volcanic eruptions because the coronene anomaly was formed by abnormally high temperature combustion," says professor Kaiho. "High temperature magma or asteroid/comet impacts can make such a coronene enrichment.

From the volcanic aspect, this could have occurred because of the higher temperature combustion of living and fossil organic matter from lava flows and horizontally intruded magma (sill) into the sedimentary coal and oil. The different magnitude of the two coronene-mercury enrichments shows that the terrestrial ecosystem was disrupted by smaller global environmental changes than the marine ecosystem. The duration between the two volcanic events is tens of thousands of years."

Huge volcanic eruptions can produce sulfuric acid aerosols in the stratosphere and carbon dioxide in the atmosphere, which causes global climate changes. This rapid climate change is believed to be behind the loss of land and marine creatures as seen in Fig. 1.

Coronene is a highly condensed six-ring polycyclic aromatic hydrocarbon, which requires significantly higher energy to form as compared to smaller PAHs. Therefore, high temperature volcanic combustion can cause the coronene enrichments. This means that high temperature combustion of hydrocarbons in the sedimentary rocks by lateral intrusion of magmas formed CO₂ and CH₄ causing high pressure and eruption to induce global warming and the mass extinction. The coronene-mercury concentration firstly evidenced that volcanic hydrocarbon combustion helped contribute to the extinction through global warming.

Kaiho's team is now studying other mass extinctions in the hopes of further understanding the cause and processes behind them.

原始文章：Kunio Kaiho, Md. Aftabuzzaman, David S. Jones, Li Tian. Pulsed volcanic combustion events coincident with the end-Permian terrestrial disturbance and the following global crisis. Geology, 2020; DOI: 10.1130/G48022.1

引用自：Tohoku University. "Large volcanic eruption caused the largest mass extinction."